Aw, don't give up. :c What you do is a really nice thing and who knows how many authors might have improved because of your advice!

Concrit is hard to give and I'm guilty of not offering it often. It can be scary pointing out things to another person - I worry about being wrong or upsetting someone needlessly.

Glad to see there are some nice people here offering you help! Though have you tried asking specifically for concrit from friends or fandom acquaintances or betas? They might feel comfortable enough to point out weaknesses in your work. I know if a friend asked I'd be honest; the last thing I'd want is for someone I care for to put out work that has grammar mistakes, plot holes, etc.